Modem communication over telephone lines has gained widespread usage with the growing number of multi-media applications implemented using personal computers. Personal computers (PCs) now perform many different types of data transfers over telephone lines, such as accessing the "Internet," communicating data files between PCs, sending and receiving facsimiles, and acting as an answering machine by sending and receiving voice information over the telephone lines. Modems are used to convert between a computer's digital signals and analog signals that can be carried on the telephone network's analog transmission lines.
In using a modem to interface a telephone line for such data transfers, there are several telephone line conditions that can adversely affect the performance of the modem and/or the telephone connection. For instance, in households having multiple telephone extensions connected to a single telephone line, it is possible for a person to try to establish a modem connection with the telephone line while another person in the household is already talking over the telephone line using another telephone extension. If the telephone line is already in use, the person on the telephone will hear annoying "popping" sounds generated by the modem trying to go off-hook in order to dial out. Further, most modems are configured to repeatedly reattempt to seek a dial tone to thereby enter the off-hook mode after failed previous attempts. This results in the annoying "popping" sounds being periodically heard by the persons speaking on the telephone while the modem attempts to dial out. There is a need for a modem system which recognizes when a telephone line is already in use, so that it does not attempt to enter the off-hook mode at that time.
Further, when a modem is used in connection with performing answering machine functions, several difficulties may arise. Once the modem receives an incoming telephone call and enters an answering machine mode, such as commencing with its outgoing message or recording an incoming message, it is possible for a person to pick-up the telephone extension during the progression of the answering machine mode. This may often occur when a person hears an incoming phone call but is not able to pick-up the telephone extension until after the modem has already initiated the answering machine mode. There is a need for a modem which recognizes when the telephone extension goes off-hook after the modem has already entered the answering machine mode, so that the modem can exit the answering machine mode and allow normal telephone conversation to occur over the telephone extension.
Also, most modems communicate by negotiating commands between the calling and receiving modems. Once the calling modem has completed its transmission, the calling modem typically sends a command to the receiving modem indicating that the transmission is complete. The receiving modem then knows to go on-hook (hang-up). However, when receiving voice information while operating in an answering machine mode, modems will not receive such commands over the telephone line indicating that the voice message has been completed. There is a need for a modem which recognizes when the party on the remote end of the telephone line connection goes on-hook, in order to notify the modem in answering machine mode to stop recording.